1. Field of the Invention
The present invention relates generally to the use of superconducting wires and more particularly relates to methods of joining such superconducting wires.
2. Description of the Prior Art
Superconductivity is the phenomenon where, when a particular material, such as a wire, is subjected to successively colder temperatures, it undergoes a state transition where all electrical resistance disappears, i.e. the material can conduct electricity without generating any heat, i.e., loss of energy. At this stage the material is said to have become a superconductor. This transition only occurs in specific metals/alloys and compounds. The state of superconductivity for a given superconductor is a function of its temperature, background magnetic field, and transport electric current. A superconductor carries current without resistance when it is below its critical temperature (Tc) and is in an environment, often referred to as background, where the magnetic field is less than its critical magnetic field (Bc). The limit of superconducting transport current in a superconductor depends on how much the operating temperature (To) and background magnet field (Bo) are below its Tc and Bc. The lower the To and Bo, the higher the limit in transport current.
There are currently two known types of superconductors: Type I and Type II. Compared to the Type I, the Type II superconductors carry more current at higher temperatures and exhibit an xe2x80x9cupper critical magnetic fieldxe2x80x9d (Bc2) which depending on the material can be as high as a few tens of tesla (T). [see for example, Stability of Superconductor, by Lawrence Dresner, Plenum Press, New York, N.Y., 1995, Chapter 1]. Practically all superconductors in commercial use are Type II.
A useful class of product made of superconductors is superconducting wires. Superconducting wires, and cables made from them, are widely used in fabrication of electromagnets, hereafter referred to as magnets, that generate magnetic fields higher than 1 tesla.
Magnets are produced by winding wires in various coil geometries. When electric current passes through the winding, a magnetic field is produced. When the wire is a superconductor, no electric power is lost in the magnet and it is called a superconducting magnet.
A current market for superconducting magnets is in devices used for Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR) spectroscopy. These devices require constant and stable magnetic fields.
A constant and stable magnetic field can be produced by flow of current through a superconducting magnet in which the ends of the superconducting wires that make up the magnet are joined together by a superconducting joint. In an ideal superconducting joint, the transport current from one wire must enter into another without electrical resistance. In such a case, current circulates in the windings of the superconducting magnet without appreciable loss of energy in the magnet, or the joint, and therefore the magnetic field that the magnet produces remains constant and the superconducting magnet is said to be in a xe2x80x9cpersistent modexe2x80x9d providing the desired constant and stable magnetic field. Thus, superconducting joints are vital components of superconducting magnets that are used in MRI and NMR devices.
For reasons that relate mainly to stability of superconductors, superconducting wires used in most magnet application are multifilamentary (MF) composites [see for example Stability of Superconductor, by Lawrence Dresner, Plenum Press, New York, N.Y., 1995]. In a multifilament superconductive wire the superconducting current is carried by superconducting filaments.
FIG. 1 illustrates a cross-section of superconducting filaments 24 spaced apart from one another within matrix 28 forming multifilamentary wire 20 such as used in most superconducting magnets in use today which filaments can be made from Niobium-Titanium (Nbxe2x80x94Ti) alloy. The Tc and Bc2 for Nbxe2x80x94Ti are about 10 Kelvin (K) and about 10 T, respectively. Nbxe2x80x94Ti alloy is ductile and basically insensitive to strain and its use in fabricating MF wires, and subsequently in a magnet, is straightforward and comparatively less expensive than other materials.
Superconducting magnets for operation at fields higher than about 10 T rely principally on the use of type A15 superconductors. Among the A15 superconductors the Nb3Sn based wires are most practical for large scale production. This is basically due to the fact that fabrication of Nb3Sn conductors is less complicated, and more economical than others. Almost all operating A15 magnets to date have used Nb3Sn conductors. The Tc and Bc2 for Nb3Sn are about 18 K and about 23 T, respectively. With proper alloying of the Nb3Sn phase, for example by Ta, its critical properties can be improved. Other A15 superconductors such as Nb3Al that have better superconducting properties than Nb3Sn are under development.
Nb3Sn, like other A15 type superconductors, is an intermetallic compound and is inherently brittle. Therefore Nb3Sn does not lend itself to normal conductor fabrication methods where a given material undergoes significant plastic deformation. For most applications in magnet technology, Nb3Sn superconductors are produced by a two-step process in which a multifilamentary composite wire that contains Nb and Sn in separate regions is formed into wire and then, during a subsequent reaction heat treatment at, for example, 650C-750C, the Nb3Sn is formed by solid state reaction. Fabrication of Nb3Sn MF wires and their use in magnets is relatively more difficult and expensive than MF Nbxe2x80x94Ti wires.
A high field superconducting magnet for xe2x80x9chigh endxe2x80x9d NMR spectrometer, for example 14T for a 600 MHz device, is typically comprised of a number of nested coils (solenoids). Please refer, for example, to J. E. C. Williams, S. Pourrahimi, Y. Iwasa, and L. J. Neuringer,xe2x80x9cA 600 MHz Spectrometer Magnet,xe2x80x9d IEEE Trans. Vol. Mag-25, pp. 1767-1770, 1989. Currently, for both economy and practicality, a high field superconducting NMR magnet uses a few coils that use Nbxe2x80x94Ti MF wires and a few coils that use Nb3Sn MF wires. The Nbxe2x80x94Ti coils combine to produce a field of up to about 10T and the Nb3Sn coils add the remaining increments of the magnetic field. FIG. 2 shows a simplified drawing of the nested coils of Nbxe2x80x94Ti 12 and Nb3Sn 14 that make up such a high field NMR magnet. A typical high field NMR magnet requires a number of superconducting joints 16 that connect through superconducting connectors 18 the nested coils to form a single unit magnet system. Magnets used in MRI devices also require superconducting joints that connect the multiple superconducting magnet modules of an overall superconducting magnet systems. Superconducting joints are also key components of xe2x80x9cpersistent switchesxe2x80x9d that are used in both NMR and MRI devices. A description of a persistent switch is given for example in Superconducting Magnets, M. N. Wilson, Oxford University Press, New York, N.Y. (1983), Chapter 11.
With improvements in economy of fabrication of A15 superconductors and the advantage that they can operate at relatively higher temperatures, next generation high field NMR magnets, or MRI magnets, may use A15 superconductors and coils exclusively.
In most conventional MF superconducting wires 20, as for example in FIG. 1, the superconducting filaments 24 are disconnected from one another for the most part. Therefore, to fabricate a superconducting joint between two MF wires often superconducting filaments 24 of the wires 20 are accessed at the wire ends by separating filaments 24 from their matrix 28. This is often done by dissolving matrix 28. In a conventional superconducting joint between two MF superconducting wires the current transfers from the filaments of one wire to the filaments of the other wire, at wire ends, through a superconducting medium. Such connection can be achieved, for example, by dissolving the metallic matrix at the wire ends and encapsulating the filaments of the wires in a superconducting solder (medium). See for example C. A. Swenson, and W. D. Markiewicz, xe2x80x9cPersistent Joint Development for High Field NMRxe2x80x9d, IEEE Transaction on Superconductivity, Vol. 9, No. 2, pp. 185-188, 1999. In yet another joint example, the matrix is dissolved and then the filaments of the wires are brought into direct contact and are then kept under compression. In yet another example, the matrix is dissolved and then the filaments of the wires are brought into contact with a superconducting medium that is a composite of consolidated Nb and Sn powders. Reference is made to J. E. C. Williams, A. Zhukovsky, R. Derocher, xe2x80x9cSuperconducting Joints With Niobium-Tin,xe2x80x9d U.S. Pat. No. 5,290,638, 1994. In this design, after making the connections between the filaments and the Nbxe2x80x94Sn composite, the composite needs to be heat treated to form the Nb3Sn superconducting phase of the medium within the composite.
Superconducting joints are also useful in applications other than joining superconducting wires in magnets or coils. They can be used to join wires to produce a relatively longer piece of superconducting wire, or to repair a broken piece of superconducting wire.
Often, the superconducting properties of the materials used in a joint dictate limitations in the design of the overall magnet including: 1) operating temperature; 2) location of the joint (background field condition); and 3) overall magnet assembly procedure. There is a need in the art for superconducting joints with better properties that can overcome the preceding limitations. In this patent application, Nb3Sn is discussed only as one example of a superconductor and the principals of invention can apply to all MF superconductive wires.
In this invention the MF superconducting wires and joints are designed and produced to provide high performance relative to current transfer at the surface of superconducting wires.
A key factor in the functionality of joints in this invention is that the superconducting medium connecting separate superconducting wires is a multifilamentary medium with interconnected filaments. In the joints of this invention, preferably but not necessarily, the MF superconducting wires are produced in a way that the individual filaments are substantially interconnected. FIG. 3 shows the cross section of a MF Nb3Sn superconducting wire 22 with interconnected filaments 24. When such superconducting filaments 24 are interconnected and are made to appear at the surface of the wire, electrical contact with filaments 24 on outer surface 26 of the wire in effect results in contact with substantially all filaments 24. Therefore, superconducting joints can be produced by making contact with filaments 24 on the wire surface 26. The present invention allows the fabrication of superconducting joints between two such similar MF wires, for example, Nbxe2x80x94Ti to Nbxe2x80x94Ti, or Nb3Sn to Nb3Sn, or dissimilar wires, for example, Nbxe2x80x94Ti to Nb3Sn.
An important implication of using wires with interconnected filaments is that when several wires of this type are joined and compacted, the filament inter-connectivity extends over the interior surface of the wire bundle, and all filaments of all wires may be accessed by contact with the exterior surface of the wire bundle. This feature is useful for making joints on cables of superconducting wires. Hereinafter all references to xe2x80x9cwirexe2x80x9d should be considered to include both wire and cable.
Another advantage of this approach is that the bridge can be a multifilamentary superconducting medium similar to the wires to be joined. This feature allows the superconductor joint to operate in background conditions that are determined by the properties of the multifilamentary superconductors. In contrast, for example, when a superconducting solder is used, the background condition is determined by the properties of the solder.
Therefore it is an object of this invention to provide a superconducting joint between a multifilamentary superconducting wire engaged to a superconducting wire having interconnected superconducting filaments disposed within a superconducting medium. The interconnected filament superconducting wire can have two ends and an outer surface in electrical contact with said wire containing superconducting filaments. Such interconnected filament superconducting wire can be produced by many processes including, but not limited to, a powder metallurgy process and a cable-in-tube process. A superconducting. bridge can be disposed around portions of said superconducting wires wherein said connection between said superconducting wires relies both on contact between said ends of said wires and said outer surfaces of said interconnected filament superconducting wire contacting said bridge. The interconnected filament superconducting wire can be comprised of a plurality of irregular, and irregularly dispersed, superconducting filaments within a superconductive matrix medium with certain of said filaments extending to the outer surface of said wire and contacting other of said filaments within the body of said wire. The superconducting bridge and medium-containing wires can be held under compression within a mechanical fixture including, but not limited to, a shrink-fit collar.
Thus a method of producing a superconductive joint between two superconducting wires is disclosed comprising the steps of providing a superconducting medium with multiple dispersed interconnected filaments therein and engaging said superconducting medium with at least two MF superconducting wires to form a superconductive joint. Such method can also include the step of engaging said superconducting wires with multiple irregularly dispersed interconnected filaments. When using such superconducting wires with multiple irregularly dispersed interconnected filaments, a superconducting joint can be produced by bringing the wire ends into direct electrical contact. Such method can also include the step of engaging said superconducting wire with multiple irregularly dispersed interconnected filaments indirectly to said other superconducting wire to form a superconducting joint. The method further includes the steps of providing said interconnected filaments within and on the outer surface of one of said wires and contacting by a bridge medium said filaments on said outer surface of said wire.